Improve locking for real this time

[djc]

• Push the lock into the ServiceAccount impls, which means the static data required to get a new token isn't locked
• Let ServiceAccount::refresh_token() return a clone of the new token so no extra locking is required
• Now no locks need to be held across await points!
• Since the locking should be read-heavy, change from a Mutex to an RwLock

[djc]

(And this time I checked that my application still compiles! Should probably add some tests...)

[hrvolapeter]

Thanks, for the fix. I was also thinking about the tests but I think in this case anything else than integration tests doesn't bring much value but those are also hard to defined and run